The present invention relates to a method for converting L-lysine.sup.. 2HCl to L-lysine.sup.. HCl.
L-lysine is one of the nutritionally essential amino acids. Its widespread use as a dietary supplement has unfortunately been inhibited by the difficulty of synthetically producing it economically and efficiently.
It is known to produce lysine by hydrolysis of certain precursors with hydrochloric acid. It has been found that in order to obtain a good rate of hydrolysis, it is necessary to use at least 2 mols HCl per mol precursor. As a result, the lysine produced is present in the reaction mixture as a dihydrochloride salt, rather than as the desired monohydrochloride salt.
A variety of methods have been suggested for removal of the extra HCl. In accordance with one method, as disclosed in U.S. Pat. No. 2,876,218, lysine dihydrochloride is first isolated by hydrolyzing .alpha.-amino-.epsilon.-caprolactam and subjecting the hydrolysis mixture, which contains lysine dihydrochloride and hydrochloric acid, to a vacuum distillation, and subsequently the lysine dihydrochloride is converted to lysine monohydrochloride by use of pyridine in an alcoholic medium. A similar method of conversion is disclosed in U.S. Pat. No. 2,564,649. In the latter case the lysine precursor is 5 (.alpha.-aminobutyl) hydrantoin hydrochloride. The recovery of the alcohol and pyridine used as well as of the hydrochloric acid split from the lysine dihydrochloride requires a number of costly operations. This process suffers from the decided disadvantage that the separation steps, noted above, are expensive and time-consuming and the process, therefore, is hardly usable for the industrial-scale preparation of lysine monohydrochloride from .alpha.-amino-.epsilon.-caprolactam. Additionally, the pyridine weak base hydrochloride is formed as a byproduct and must also be removed.
U.S. Pat. No. 2,579,283 teaches the production of lysine by hydrolysis of a polymeric hydantoin followed by the conversion of lysine dihydrochloride to lysine monohydrochloride by passing an aqueous solution of the dihydrochloride over an anion exchange resin. The resin is then regenerated to yield a chloride solution; however, there is no way to recover the processing agents, e.g. ammonium hydroxide, sodium hydroxide or other bases, which were employed to regenerate the resin. When it became known to use L-aminocaprolactam as a satisfactory lysine precursor, the use of an ion exchange resin was expanded in U.S. Pat. No. 3,576,859 to include the recovery and reutilization of the processing agents. In this patent, the lysine dihydrochloride, produced from the hydrolysis of .alpha.-amino-.epsilon.-caprolactam, is passed through an anion exchanger to remove HCl. Then, the HCl bound to the resin is removed by formation of a salt with additional .alpha.-amino-.epsilon.-caprolactam and the salt is then reused in the hydrolysis step. However, both of these patents have the disadvantage of necessitating bulky, costly and time-consuming anion exchange resin equipment. Moreover, the lysine monohydrochloride thus formed is obtained as a relatively dilute aqueous solution which must be further processed to recover the lysine monohydrochloride in the desired form.
Additionally, all these methods require that the lysine precursor be completely hydrolyzed to lysine-2HCl to prevent the occurrence of any yield loss and/or product contamination during the subsequent conversion steps and/or recycling operations. Since the hydrolysis proceeds at an exponentially decreasing rate, the necessity for hydrolysis of the last 5-10 percent of the lysine precursor renders the operation more difficult and time-consuming.
Not withstanding the various prior methods, the method of the present invention affords a definitely advantageous alternative by producing an economical, efficient and relatively fast method of converting lysine dihydrochloride to lysine monohydrochloride.
The method of the present invention is also advantageous in that it produces lysine monohydrochloride in a system which does not require complete hydrolysis of the lysine precursor.
Moreover, the present invention provides a method for the conversion of lysine dihydrochloride to lysine monohydrochloride in which a lysine precursor is used as the neutralizing agent thereby ensuring the complete utilization of all the processing materials.